eas 384shallowfoundation2011

7/31/2019 EAS 384ShallowFoundation2011

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EAA 384

CONSTRUCTION TECHNOLOGY

FOUNDATIONSHALLOW FOUNDATION

DR. NORAZURA MUHAMAD BUNNORI



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Topic Outcomes

At the end of this topic, students able to:

1. Define the function of the foundation.

2. Identify the different types of shallow foundations.

3. Explain the activities sequence of shallow foundations.

4. Compare the advantages and disadvantages of the shallow foundations.

5. Recognize the drawings associated to shallow foundations.


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Introduction

The foundation of the structure supports the weight of the structure and its applied

load.

In broad sense, the term foundation includes the soil or rock upon which a

structure rest.

Unlike superstructure, substructure components always associated to ground or

anything beneath it.

As foundation lies at the end of the chain of the load path, the design, construction

and supervision of foundation work requires more extra causes.

Failure to substructure/foundation can either be due to excessive settlement,

overturning, sliding (geotechnical aspect) OR the structure itself.

The of any is to safely sustain and transmit to the ground onwhich it rests the combined dead, imposed and wind loads in such a manner as not

to cause any settlement or other movement which would impair the stability or

cause damage to any part of the building.



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Stresses imposed by structures

Abutments and piers may have shallow or deep foundations


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Shallow Foundation

Shallow foundations, sometimes knows as non-piled foundations

divided under 3 specific types:

Pad footings either isolated or combined

Strip footing

Raft footing

In each case, the choice is related to: Ground condition-adequate soil bearing capacity

Loading that have to be supported-normally of low column load



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Pad Footing

While strip footings are best designed to support linear load of low intensity,

pad foundations are designed to support high loads over limited area.

Such foundations are common where a structural form brings loads to the

ground by way of columns. As such, the applicable to reinforced concrete,

precast concrete and structural steel design solutions.

Depending on ground conditions, pad will be found in a wide variety ofstructures-warehouse, low rise industrial plants requiring large clear areas and

low-to-mid rise buildings with favourable ground below.

The plan shape of the pad foundation normally square with smallest dimension

to be 600mm x 600mm and can be up to 3m x 3m or more. Thickness starts

with 250mm. At some occasion, the plan shape can be rectangular, normally to resists

overturning due to lateral load.



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Pad Footing

Elevation profile

Single/spread footing

Steeped footing

Slope footing

Precast concrete (pcc) column

Footing with pedestal


x

x

SQUARE PAD FOOTING

x

y

RECTANGULAR

PAD FOOTING


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Pad Footing


SINGLE

COLUMNSTUMP

STEEPED

COLUMN

STUMP

1st Layer

2nd Layer

COLUMNSTUMP

SLOPE

PRECASTCOLUMN

GROUT

WITH PCC COLUMN

CONCRETE PEDESTAL

NORMALLY STEEL COLUMNBASE PLATE & BOLT

WITH PEDESTAL


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Pad Footing

Excavate pad foundation

Provide support to excavation (more than 1.2 meter or where week soils

overlay)

Compact base, place hardcore & compact, place sand blinding/lean

concrete. Check column center

Fix formwork and reinforcement

Provide adequate stump starter bar and place concrete. Make sure dry

condition during concreting. Use pump if necessary.

Remove formwork and support materials

Backfill, compact and dispose surplus

Check offset and tolerances



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Pad Footing



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Pad Footing

Suitable for low rise building with relatively high column load

Very economical compared to pile foundation

Less excavation area (but deeper excavation required)

Lesser reinforcement and concrete usage

Uniform pad size may allow repetitious usage of formwork

Require relatively high soil bearing pressure especially for 3-4 storeys

building (75-150kN/m2 for 1-2 storey & 200-250kN/m2 for 3-4 storeys to

become economical)

May require protection to excavation work

Deeper than 1.5m is not economical and dangerous

Mat require dewatering (pumping out ground water)



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Pad Footing

When designing a foundation, the area of the base must be large enough

to ensure that the load per unit area does not exceed the bearing capacity

of the soil.

The ideal situation when considering column foundation is to have asquare base with column positioned centrally.

However at times, ideal condition is not always possible.

When two columns are closely placed, and the load carried by each

column is significant, two isolated pad footing may overlapped each other.

If the base area is constraint by the boundary or by other adjacent closelyspaced columns, then combining these isolated pads (hence the term

combined foundation) may be solution.



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Pad Footing

Plane shape

Rectangular where column load is almost equal

Trapezoidal where one column load carries the heavier load

Regardless of the plane shape, normally the center of gravity of the

combined pad footing is designed to be coincide with the center of gravity

of the loads (from both columns)

This will improve the possibility of eccentric connection that may lead tounacceptable differential settlement.



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Pad Footing



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Pad Footing



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Strip Footing

As the name implies, strip footings are long continuous strip, usually of concretecreated in the ground at a suitable depth to provide adequate support for the

loads brought upon them.

It is most suited to support load bearing wall of low rise buildings (linear load)

rather than individual column.

Just like any other shallow foundations, concrete ground floor shall be

supported by the soil and not attach to the load bearing/column. This will

reduce loading to the wall/column as well as to the strip foundation.

There are two construction methods that can be adopted for strip footings

namely:

Traditional method common in Malaysia (reinforced)

Trench fill method mass concrete.



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Strip Footing



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Strip Footing



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Strip Footing

Mark the working area, remove topsoil and store on site, check reducedlevel.

Excavate trenches

Support excavated trenches if more than 1.2m deep or supporting

earth wall is week (loose sand)

Prepare formation of trenches and compact the base

Lay sand blinding or lean concrete (normally 50mm thick)

Fix reinforcement as specified.

Concrete to footing extending the starter for column (if column is used).

Minimum thickness is about 200mm. Brickwork to ground level or concrete column to ground level.

Backfill to ground level or formation level of floors as specified

Remove surplus excavation off site.



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Strip Footing

Mark the working are, excavate to reduce level after removal of top soil

Excavate trenches and position duct for service entry (if any).

Fill concrete (mass concrete) in trenches

Remove excavated material off site

Can be constructed shallower than pad foundation, less excavation

Only one layer of reinforcement is required

Suitable for low ground bearing pressure compare to pad foundation

Suitable for linear load (load bearing wall)

Extra concrete volume compares to pad foundation

Not suitable for supporting column with high load

Not economical (in terms of excavation work) if the strip foundation is to be located

deeper into ground.



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Raft Footing

The name gives the lead to this type of foundation. In broad terms, the raft foundation is designed to

float on poor ground while distributing local heavy loads which come down upon it, to an acceptablefinal ground pressure.

One unique features of raft foundation is that it serves both as foundation and slab and does not require

ground beams except at perimeter. Even the beams at perimeter are not structural beams.

Strip top soil

Excavate to reduce level (minor)

Compact base

Lay hardcore (about 150-200mm) and compact (well compacted hardcore)

Install service entries, duct, pipe work etc under the floor area

Fix external formwork and at drop are (if any)

Install DPC (damp Proof Course)

Install reinforcement and starter bars

Lay concrete

Remove surplus materials



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Raft Footing

In a case where floor drops need to be provided, 3 methods are normally adopted in the

construction namely: Folded soffit of raft varies, equal raft thickness, extra work

Tapered soffit or raft at the same level, extra concrete, easier to work but may

cause unnecessary thickness of raft.

Topping soffit same level, topping with mass concrete, quite economical.



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Raft Footing

No ground beam construction except at perimeter (if required) due to significantdifference between platform level and finished ground floor level

Lesser usage of formwork at ground level, normally at perimeter or at drop area.

Lesser excavation at ground level, no protection to excavation work.

May reduce construction time because completion of raft foundation means completion

of ground slab and ground beam. Most suitable for soil with low bearing capacity (about 30N/mm2)

Normally require two layers of reinforcement and this may increase construction cost.

Not economical for medium rise structure (although adequate soil bearing can be

achieved) because it will increase the raft thickness.

Careful planning on the work sequence required where services 9water, sewerage etc)

need to be laid under the raft. Maintenance of such services also becomes difficult after

completion

Normally requires construction joint (extra cost)



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Drawing associated to shallow foundations

Normally footing layout and marking of different footing sizes

Easier to prepare table showing dimensions for all type (L x B x H) and reinforcement

Provide typical plan showing L, B, H and arrangement of reinforcement

Provide typical cross section highlighting other requirements.

Layout & location of cross section

Typical cross section (may be more than one cross section required) showing depth of

foundation, thickness, width, reinforcement, blinding etc.

Layout and location of cross sections

Cross section drawings (more cross section required in x and y directions,) at

drop areas, perimeter, topping area etc




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Examples:

Shallow foundation for bridge abutments



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Examples:

Shallow foundation for bridge abutments



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Examples:

Combined footings



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Thank You


eas 384shallowfoundation2011

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